Ingela Nyström

Computing skeletons in three dimensions

Skeletonization will probably become as valuable a tool for shape analysis in 3D, as it is in 2D. We present a topology preserving 3D skeletonization method which computes both surface and curve sk ...

Curve skeletonization of surface-like objects in 3D images guided by voxel classification

Skeletonization is a way to reduce dimensionality of digital objects. Here, we present an algorithm that computes the curve skeleton of a surface-like object in a 3D image, i.e., an object that in ...

Efficient shape representation by minimizing the set of centres of maximal discs/spheres

Abstract Efficient shape representations are important for many image processing applications. Distance transform based algorithms can be used to compute the set of centres of maximal discs/spheres, that represents a shape. This paper describes a method that reduces this set, under the constraint that the shape can be exactly reconstructed using the reverse distance transformation. The reduced set can be used in the same ways as the “standard” set, e.g. for efficient storage, segmentation into parts of different thickness, shape manipulation, and skeletonization, all in 2D and 3D.

Measurements of digitized objects with fuzzy borders in 2D and 3D

The results of our investigation of several measurements on digitized 2D and 3D objects with fuzzy borders are presented. The performance of surface area, volume, and roundness measure estimators for digitized balls with fuzzy borders is analyzed. The method we suggest provides significant improvement in precision, compared to analogous estimation results obtained on a crisp (hard) segmentation, especially in the case of low resolution images.

On Reversible Skeletonization Using Anchor-Points from Distance Transforms

In many applications thinning of objects is of great interest. We here present a skeletonization algorithm that is based on the idea of iteratively thinning the distance transform of an object layer by layer until either an anchor-point is reached or the connectivity breaks. Our algorithm is general in the sense that any metric and any connectivity can be used. Also, it is based on ideas that are not specific for 2D. The properties of the resulting skeletons are evaluated according to the “Lee?Lam?Suen properties.”

Tripeptide Interference with Human Immunodeficiency Virus Type 1 Morphogenesis

ABSTRACT Capsid assembly during virus replication is a potential target for antiviral therapy. The Gag polyprotein is the main structural component of retroviral particles, and in human immunodeficiency virus type 1 (HIV-1), it contains the sequences for the matrix, capsid, nucleocapsid, and several small polypeptides. Here, we report that at a concentration of 100 μM, 7 of 83 tripeptide amides from the carboxyl-terminal sequence of the HIV-1 capsid protein p24 suppressed HIV-1 replication (>80%). The three most potent tripeptides, glycyl-prolyl-glycine-amide (GPG-NH2), alanyl-leucyl-glycine-amide (ALG-NH2), and arginyl-glutaminyl-glycine-amide (RQG-NH2), were found to interact with p24. With electron microscopy, disarranged core structures of HIV-1 progeny were extensively observed when the cells were treated with GPG-NH2 and ALG-NH2. Furthermore, nodular structures of approximately the same size as the broad end of HIV-1 conical capsids were observed at the plasma membranes of treated cells only, possibly indicating an arrest of the budding process. Corresponding tripeptides with nonamidated carboxyl termini were not biologically active and did not interact with p24.

Synthesising Objects and Scenes Using the Reverse Distance Transformation in 2D and 3D

The reverse distance transformation has proved useful in image synthesis. This paper describes how digital objects are created from a number of seed labels in an image. The shape of the obtained objects depends on the metric used. In 2D the Euclidean and the 3-4 metrics are mentioned, and in 3D the D6, the D26, and the 3-4-5 metrics are discussed. The proposed method has no need of expensive CAD systems. It is an excellent image synthesising tool when developing image processing algorithms, i. e. shape quantification, visualisation, scene analysis and range imaging, as the obtained objects are well-defined in the image. The method is most advantageous in 3D, as there is an increasing need for volume images, but synthesising objects in 2D can also be useful.

A graph-based framework for sub-pixel image segmentation

Many image segmentation methods utilize graph structures for representing images, where the flexibility and generality of the abstract structure is beneficial. By using a fuzzy object representation, i.e., allowing partial belongingness of elements to image objects, the unavoidable loss of information when representing continuous structures by finite sets is significantly reduced, enabling feature estimates with sub-pixel precision. This work presents a framework for object representation based on fuzzy segmented graphs. Interpreting the edges as one-dimensional paths between the vertices of a graph, we extend the notion of a graph cut to that of a located cut, i.e., a cut with sub-edge precision. We describe a method for computing a located cut from a fuzzy segmentation of graph vertices. Further, the notion of vertex coverage segmentation is proposed as a graph theoretic equivalent to pixel coverage segmentations and a method for computing such a segmentation from a located cut is given. Utilizing the proposed framework, we demonstrate improved precision of area measurements of synthetic two-dimensional objects. We emphasize that although the experiments presented here are performed on two-dimensional images, the proposed framework is defined for general graphs and thus applicable to images of any dimension.

Shape signatures of fuzzy star-shaped sets based on distance from the centroid

We extend the shape signature based on the distance of the boundary points from the shape centroid, to the case of fuzzy sets. The analysis of the transition from crisp to fuzzy shape descriptor is first given in the continuous case. This is followed by a study of the specific issues induced by the discrete representation of the objects in a computer.We analyze two methods for calculating the signature of a fuzzy shape, derived from two ways of defining a fuzzy set: first, by its membership function, and second, as a stack of its α-cuts. The first approach is based on measuring the length of a fuzzy straight line by integration of the fuzzy membership function, while in the second one we use averaging of the shape signatures obtained for the individual α-cuts of the fuzzy set. The two methods, equivalent in the continuous case for the studied class of fuzzy shapes, produce different results when adjusted to the discrete case. A statistical study, aiming at characterizing the performances of each method in the discrete case, is done. Both methods are shown to provide more precise descriptions than their corresponding crisp versions. The second method (based on averaged Euclidean distance over the α-cuts) outperforms the others.

A 3d live-wire segmentation method for volume images using haptic interaction

Designing interactive segmentation methods for digital volume images is difficult, mainly because efficient 3D interaction is much harder to achieve than interaction with 2D images To overcome this issue, we use a system that combines stereo graphics and haptics to facilitate efficient 3D interaction We propose a new method, based on the 2D live-wire method, for segmenting volume images Our method consists of two parts: an interface for drawing 3D live-wire curves onto the boundary of an object in a volume image, and an algorithm for connecting two such curves to create a discrete surface.